1. Field of the Invention
This invention relates to a press machine controller for controlling a press machine including a servo motor which drives a slide via a reduction mechanism with the reduction ratio changed in accordance with the position of the slide.
2. Description of the Related Art
Conventionally, a mechanical press in which press forming time can be shortened by operating a slide at high speed can be used as a press machine. Japanese Unexamined Patent Publication No. 10-249590 discloses a mechanical press having a link mechanism. The slide of the press machine disclosed in Japanese Unexamined Patent Publication No. 10-249590 moves up and down with the motion of a servo motor transmitted through a link mechanism.
In the press machine disclosed in Japanese Unexamined Patent Publication No. 10-249590, the reduction ratio of the link mechanism for the motor is changed in accordance with the position of the slide. Specifically, the reduction ratio is maximized when the slide reaches the bottom dead center, and at this point, the largest torque is required for the motor.
In the case where the slide simply continues to move through the bottom dead center, the inertia of the motor and the dynamic friction acting on the link mechanism, etc., eliminate the need of a large torque.
However, in the case where the press machine is stopped with the slide located at the bottom dead center for the purpose of confirming the operation of the press machine or otherwise, the inertial force fails to work and the static friction acts on the link mechanism, etc. Therefore, a large torque is required to restart the press machine.
In order to generate a large torque normally not required, the size of the servo motor of the press machine has to be large. However, a large servo motor is expensive, resulting in increased production cost of the press machine.
This invention has been achieved in view of this situation, and the object thereof is to provide a press machine controller whereby the press machine, even if stopped with the slide located at the bottom dead center, can be restarted with a small torque.